In today's wireless communication networks, Public Land Mobile Networks (PLMNs) have been established to provide land mobile telecommunications service to the public. Generally, each service provider operates its own PLMN, albeit a number of service providers may share some network equipment. The number of PLMNs in a country may be as little as a single PLMN in a small country, to as many as twenty or more in a large country like India. A PLMN may provide service in one, or a combination, of frequency bands. Different PLMNs present in a common geographical region or cell may operate using the same or different access technologies such as time division multiplexed (TDM) based Global System for Mobile communications (GSM), or, CDMA based systems such as CDMA2000 or WCDMA.
Most of the time, a mobile terminal (MT) communicates via its own service provider's PLMN, which is called the Home PLMN (HPLMN). Occasionally, the MT cannot acquire service on the HPLMN, or it may lose service. In this case, the MT may attempt service acquisition on another PLMN, and if successful, this PLMN is designated a Visited PLMN (VPLMN). The attempt to employ a VPLMN is what is commonly known as roaming. Typically, a prioritized list of PLMNs that the MT is allowed to access is stored on a Subscriber Identity Module (SIM) card or other non-volatile memory within the MT. The entries and prioritization in the list is usually based on agreements contracted among the various service providers.
In networks that operate in accordance with the current Third Generation Partnership Project (3GPP) protocol, the MT (also referred to as User Equipment or UE) supports both automatic and manual network selection mechanisms (modes) as described, e.g., in specification 3GPP TS 22.011. In the automatic selection mode, which is the mode used the vast majority of time, the MT consults the PLMN priority list pre-established by the network operator (service provider), and attempts to register on a PLMN in accordance with the listed priority. If registration cannot initially be obtained on the HPLMN, the MT will then attempt to register on the next highest priority PLMN on the list, and so on, until service is acquired.
In the manual selection mode, the user is given an opportunity to select an available PLMN. The MT contains display functions by which available PLMNs and the selected PLMN can be indicated. For instance, in a “settings” display function, various settings of the MT may include available PLMNs which can be selected by the user.